In an apparatus using an electrographic technique such as a copy machine, printer or facsimile, a developing device contains a developer including two components, that is, toner and carrier, for developing an electrostatic latent image on a photoconductor surface. The developing device supplies toner to the photoconductor surface.
In the case of a dry developer, the toner is consumed in the development of a latent image, whereas the carrier remains in the developing device. As for the carrier, the resin coating material on the surface may be separated or toner components may adhere to the surface. Such a carrier lowers charging capability of the developer and causes degradation in image characteristics.
To deal with this, a method of supplying a carrier as well as a toner to the developing device is proposed. In this method, unlike a method of totally replacing a developer in certain timing during its life, the carrier is supplied simultaneously with the toner during normal development. The toner is consumed in the development, whereas the supplied carrier remains within the developing device. If the toner density in the developing device is to be maintained at a constant level, the bulk of the developer in the developing device is increased. The developer that overflows the developing device is discharged outside the developing device. This method is a discharge method utilizing a so-called overflow.
JP-A-7-121017 discloses a method for maintaining toner density in a developing device at a constant level, utilizing an overflow of a developer. JP-A-7-121017 discloses a configuration in which a discharge port for discharging the developer is formed in the shape of an inverted triangle in the direction of the height of the developing device.
With the configuration disclosed in JP-A-7-121017, even if the amount of developer in the developer tank is suddenly increased, the developer can be quickly discharged outside the developing device by the amount of increase. Therefore, the amount of developer in the developer tank restores a standard amount.
However, characteristics including physical properties, fluidity and bulk density of the developer change depending on the environment and life (degradation of the developer with time). JP-A-7-121017 does not disclose whether the discharge port is configured to be capable of dealing with changes in fluidity of the developer or not. That is, with the configuration disclosed in JP-A-7-121017, the developer in the developer tank cannot be maintained in a standard mount if the fluidity of the developer is lowered. The developing device cannot sufficiently supply the developer to the developing roller. Therefore, the developing device has a problem that an uneven developer layer is formed on the developing roller.
Thus, the invention provides a developing device having a structure that can efficiently discharge a developer outside.